Both the characteristic of lightning surge, and overvoltage protection technologies have been introduced in the article “Earthing system and lightning protection,” by Hitoshi Kijima, first edition, Corona Co., IEICE, Apr. 5, 2002, pp. 1-54 (Non-Patent literature 1). The technical background required for understanding the present invention will be briefly described below. FIG. 1 shows the profile S (t) of voltage or current caused by lightning surge. Generally, a direct lightning strike causes a current having a peak value of several tens of kA to flow, and an induced lightning surge causes a voltage having a peak value of several kV. Accordingly, values of voltage or current versus time are plotted in FIG. 1. In testing an overvoltage protective device, a voltage waveform having a peak value of 6 kV, a mean time to half value of 50 μsec, and a front time of 1.2 μsec is usually applied to observe the change of voltage.
Overvoltage protective components are mainly divided into two categories. One is clamping type and the other is switching type. Typical clamping type overvoltage protective components are varistors and diodes. These components are characterized by having a region in which an increase of current hardly causes voltage to vary. FIG. 2A shows a configuration for overvoltage protection using a varistor. Even when an overvoltage is applied across terminals 902 and 903, an overvoltage is not applied across terminals 904 and 905 by virtue of a varistor 901. FIG. 2B shows a configuration for overvoltage protection using a diode. In the configuration of FIG. 2B, two diodes are arranged in parallel to prevent overvoltage from being applied across terminals 944 and 945 when an overvoltage having positive polarity is applied to an terminal 942 as well as when an overvoltage having negative polarity is applied to the terminal 942. FIG. 2C shows an ideal change of voltage across indoor side terminals when a clamping type overvoltage protective component is used. In the case of an ideal clamping type overvoltage protective component, even when the outdoor side voltage exceeds operating voltage C, the indoor side voltage is approximately constant at operating voltage C as indicated by waveform 908. When the voltage across outdoor side terminals becomes lower than operating voltage C, the voltage across indoor side terminals also decreases. FIG. 2D shows the change of current flowing through a clamping type overvoltage protective component when the clamping type overvoltage protective component is used. In the overvoltage protective component, instantaneous power (voltage×current) is consumed, and there is generated a heat equivalent to energy (voltage×current×time) obtained by integrating the instantaneous power in the time domain. In the case of a clamping type overvoltage protective component, the operating voltage is relatively high, so when an extremely large current flows, its life may be affected by these instantaneous power and energy.
Switching type overvoltage protective components are represented by discharge tubes. Discharge tubes are components for overvoltage protection by a discharge phenomenon, with insulation between electrodes with gas (including vacuum), liquid, solid, or a mixture thereof. Typical discharge tubes are gas-filled discharge tubes and air gap discharge tubes. Discharge tubes are characterized in that when discharge starts, voltage significantly becomes lower. FIG. 3A shows a configuration for overvoltage protection using a gas-filled discharge tube, and FIG. 3B shows a configuration for overvoltage protection using an air gap discharge tube. FIG. 3C shows an ideal change of voltage across indoor side terminals, in the case of using a discharge tube. In the case of using a discharge tube, when an overvoltage applied from the outdoor side exceeds operating voltage B, the voltage across the overvoltage protective component terminals becomes sharply low by discharge. After the discharge starts, the voltage across the discharge tube terminals is lowered to spark-over voltage A. Waveform 918 represents the change. Thereafter when the voltage across outdoor side terminals becomes lower than spark-over voltage A, the voltage across indoor side terminals becomes the same spark-over voltage A. However, when a commercial power supply of, for example, 100 V is used, if spark-over voltage A is lower than 100 V, even after the overvoltage caused by a lightning surge disappear, discharge will continue due to the current supply from commercial power line. Accordingly, countermeasures are taken such as connecting other discharge tubes in series to raise total spark-over voltage to 100 V or higher. Such extinguishing of discharge once generated by the lightning surge and continued by the commercial power is referred to as “follow current interrupt”. FIG. 3D shows the change of current flowing through a discharge tube when the discharge tube is used. In the case of a discharge tube, when voltage is high, current does not flow through the discharge tube; when current flows, voltage lowers. Consequently, instantaneous power (voltage×current) as well as energy (temporal integration of instantaneous power) are relatively small. Accordingly, in the case of a discharge tube, even when an extremely large current flows, its life is not adversely affected to a large extent.
Varistors and diodes being used as a clamping type overvoltage protective component are typically composed of a semiconductor. Accordingly, time TC taken for a clamping type overvoltage protective component to reach its operating voltage is as small as 0.01 μsec. On the other hand, discharge tube is based on a discharge phenomenon, so time TB taken to reach its operating voltage is 1 μsec, which is relatively long.
There have been proposed various methods for combination of both switching type components and clamping type components to reduce both the response time and the voltage across indoor side terminals simultaneously. For example, FIG. 4A shows a configuration which connects in parallel a discharge tube 911 and a varistor 901 and inserts a coil 961 therebetween. FIG. 4B shows the change of voltage across terminals of the discharge tube 911, and FIG. 4C shows the sum of currents flowing through the discharge tube 911 and the varistor 901, respectively, when voltage S (t) is applied across outdoor side terminals 962 and 963 in the exemplary configuration of FIG. 4A. In the varistor 901, it takes shorter time to reach the operating voltage compared to that of the discharge tube 911. Consequently, when voltage S (t) is applied to the outdoor side, current first flows through the varistor 901. At this time, a voltage is generated across both ends of the coil 961 by the change of current. Thereafter the voltage across both ends of the discharge tube 911 becomes higher than operating voltage B, whereby the discharge tube 911 is activated. By operating in this manner, in a short response time substantially equal to the time taken for the varistor, the voltage across indoor side terminals 964 and 965 can be lowered to the operating voltage of a varistor. Then the discharge tube is activated, whereby the voltage across indoor side terminals 964 and 965 can be lowered to spark-over voltage A of discharge tube. Any large current flows through the discharge tube 911, so the life of varistor is not affected. However, as the coil 961, a self-inductance value of about 10 μH is typically required. Further, at the time of normal operation, current from commercial power line flows through the coil, so it is needed to construct the coil with a wire having a large current capacity. Consequently, the size of the coil 961 becomes large, thus causing a problem in reducing the size and cost. In the case of an overvoltage protective device in a communication system, instead of a coil, a resistor can also be used. At the time of normal operation, however, signal current or power supply current flows through the resistor at all times, thus causing a problem of voltage drop. Further, when there is a deviation of resistance values of resistors inserted to each conductor, the degree of balance between the two conductors may be deteriorated.
FIG. 5A shows an exemplary configuration which connects in series a discharge tube 911 and a varistor 901. FIG. 5B shows the change of voltage across indoor side terminals 974 and 975, and FIG. 5C shows the change of current flowing through the discharge tube 911 and varistor 901, when voltage S (t) is applied across outdoor side terminals 972 and 973 in the exemplary configuration of FIG. 5A. Until the discharge tube 911 is activated, current does not flow through the varistor 901, so the voltage across both ends of the varistor 901 remains 0. More specifically, voltage S (t) across outdoor side terminals 972 and 973 is applied across both ends of the discharge tube 911. Accordingly, in this configuration, the operation occurs when operating voltage B of the discharge tube 911 is reached. After discharge is started, current flows through the varistor 901, so the sum of spark-over voltage A of the discharge tube 911 and operating voltage C of the varistor 901 is applied across indoor side terminals 974 and 975. In this exemplary configuration, the response time is the same as that of a discharge tube. Thus it takes longer for the operation to occur. Also, while voltage is high, the overall current flows through the varistor 901, thus causing a problem of adversely affecting the life of varistor. However, after the operations occur, voltage can be adjusted to C+A, thus enabling follow current interrupt, if C+A exceed the commercial power supply voltage.
FIG. 6A shows an exemplary configuration which arranges in parallel two identical discharge tubes, and FIG. 6B an exemplary configuration which connects in series two identical discharge tubes. Generally, discharge tubes have poor polarity symmetry. More specifically, the operating voltage varies according to which of both terminals is positive. The operating voltage when voltage having positive polarity is applied to the conductor 916 side of a discharge tube 911i is referred to as Bi1, and the operating voltage when voltage having negative polarity is applied to the conductor 916 side is referred to as Bi2. When discharge tubes 9111 and 9112 are arranged in parallel as shown in FIG. 6A and voltage having positive polarity is applied to the conductor 916, the operating voltage is equal to the lower one of B11 and B21. When voltage having negative polarity is applied to the conductor 916, the operating voltage is equal to the lower one of B12 and B22. In the case of a overvoltage protective device using the discharge tube 9111 alone as an overvoltage protective device, the operating voltage is B11 when voltage having positive polarity is applied to the conductor 916, and is B12 when voltage having negative polarity is applied to the conductor 916. For example, when B11<B21<B22<B12, the operating voltages of FIG. 6A are B11 and B22. Accordingly, symmetry is improved compared to the operating voltages B11 and B12 when the discharge tube 9111 alone is used. When discharge tubes 9113 and 9114 are arranged in series as shown in FIG. 6B, again, if one having a lower operating voltage is activated, almost the entire voltage is applied to the other discharge tube, so the other discharge tube is also activated. More specifically, when discharge tubes are connected in series, again, symmetry can be improved. Even when a plurality of discharge tubes is arranged in parallel or in series, it is also possible that symmetry is not improved depending on a combination of operating voltages. In this case, however, symmetry is not deteriorated, and remains the same or is improved. Even combination of several discharge tubes without any varistors, the high response time cannot be achieved like as that of varistors.
FIG. 7 shows an example of overvoltage protective device for use in a communication system using a varistor and a three electrode discharge tube. In this overvoltage protective device, a sharp leading edge of lightning surge is absorbed by a varistor having a short response time and a low operating voltage. Subsequently, the three electrode discharge tube is activated by voltage drop caused by resistors 931 and 932, whereby a large part of lightning surge energy is absorbed. This is an example in which the configuration of FIG. 4A is improved in symmetry. When the above overvoltage protective device is used for an electric power system, the resistors 931 and 932 must be replaced with coils, thus causing a problem in reducing the size and cost.
Non-patent literature 1: “Earthing system and lightning protection,” by Hitoshi Kijima, first edition, Corona Co., IEICE, Apr. 5, 2002, pp. 1-54